Existing wind turbine designs look very much like aircraft propellers. Underlying this resemblance is the apparent assumption that the wind turbine problem—extracting energy from the wind—is the dual of the aircraft propulsion problem. Further to this assumption, existing wind turbines rotate fairly rapidly, with blade tip speeds around 200 miles per hour. Noise from the tip vortex is a necessary result. The fast moving rotor blades also pose a distinct hazard to birds and other flying creatures.
On closer examination, however, the assumptions underlying the foregoing design approach appear to be open to question. In particular, aircraft propellers are designed to convert torque from the engine into a forward-directed lift force. One byproduct of this process is “propeller wash,” a mass transfer of air from ahead of the propeller to behind it. This transfer of air appears as a wind flowing in a direction opposite to the lift force to someone standing behind the propeller of a stationary airplane.
The dual relationship considers this problem in reverse: that air motion past a stationary propeller should produce a torque on the propeller shaft, and a lift force in a direction opposite to the wind direction. Conventional wind turbines demonstrate that this approach is workable. Whether it is optimal is a different question. The high tip speed requires the blade to be twisted (airfoil chord significantly misaligned to the wind direction) so that it is not physically aimed in the direction of the wind. In a conventional design, this arrangement results in there being a particular wind speed that is optimum for the desired energy conversion. Both lower and higher wind speeds have reduced energy conversion efficiencies as seen, for example, in FIG. 1. Wind speeds near the earth's surface are quite variable at any particular location. Furthermore, higher wind speeds cause higher turbine rotation rates, resulting in a safety hazard and necessitating complexity in turbine control design to address this hazard.
Considering the goal of converting the present wind (whatever speed that is) into torque that can be used to generate electricity, conventional arrangements leave something to be desired.